Perforated trays with downcomers are one of the most commonly used internals in industrial columns for contacting a gas and a liquid. In such a column, liquid flows across the trays and down through the downcomer from tray to tray while gas permeates upwardly through the trays foaming the liquid thereon. At low liquid flow rates, a single-downcomer is usually used for each tray. But, at high liquid flow rates, as in the case of pressure distillation and absorption, one downcomer may not be sufficient to handle the liquid flow and downcomer flooding may occur. Therefore, plural downcomers have to be installed for each tray, see, for example, "Use of Multiple Downcomer Trays to Increase Column Capacity", Union Carbide Corporation, 1983. Plural downcomers are conventionally arranged to separate the flowing liquid on the tray into two, four or even six passes, i.e., separate the flowing liquid into a number of different, shorter streams, each of which flows to a different downcomer. The disadvantages for this conventional arrangement can be divided into the following three aspects: (1) The complicated structure increases construction price, (2) The short liquid passage reduces liquid/vapor contact time on each tray, and thus lowers the tray efficiency, and (3) It is almost impossible to split the flow of liquid on a tray equally into two, four or six passes, and so severe vapor/liquid maldistribution on each tray can result.
Downcomer flooding may be caused by many factors, such as high tray pressure drop, unduly large fluid friction through each downcomer, and so on. However, the frictional head loss in each downcomer entrance region is probably to be the most significant. Thus, the downcomer capacity is probably mainly limited by the entrance friction.
There is a need for a gas-liquid contacting device wherein an increase in tray capacity is achieved by an increase in the liquid downcomer handling capacity and a reduction in the frictional head loss in the entrance region thereto.